The invention concerns a photoactive component with organic layers, in particular a solar cell, consisting of a series of organic thin layers and contact layers with a doped transport layer and a photoactive layer, arranged in a pi, ni, or pin diode structure from a p, i or n layer respectively.
Since the demonstration of the first organic solar cell with an efficiency in the percentage range by Tang et al. 1986 [C. W. Tang et al. Appl. Phys. Lett. 48, 183 (1986)], organic materials have been intensively examined for various electronic and optoelectronic components. Organic solar cells consist of a series of thin layers (typically 1 nm to μm) comprising organic materials which are preferably vapor deposited in a vacuum or spin coated. The electrical contacting generally occurs via metal layers and/or transparent conductive oxides (TCOs).
A solar cell converts light energy into electrical energy. In contrast to inorganic solar cells, free charge carriers are not directly created by the light in organic solar cells, but rather excitons are initially formed, i.e. electrically neutral excitation states (bound electron-hole pairs). Only in a second stage are these excitons separated into free charge carriers which then contribute to the electrical current flow.
The advantage of such organic-based components in comparison to conventional inorganic-based components (semiconductors such as silicon, gallium arsenide) is the, in part, extremely high optical absorption coefficients (up to 2×105 cm−1), thus providing the opportunity to create very thin solar cells using a low amount of materials and energy. Further technological aspects are the low costs, the possibility of creating flexible large-area components on plastic films and the almost unlimited variations available in organic chemistry.
An option for realizing an organic solar cell already proposed in the literature [Martin Pfeiffer, “Controlled doping of organic vacuum deposited dye layers: basics and applications”, PhD thesis TU Dresden. 1999] consists of a pin diode with the following layer structure:
0. Carrier, substrate,
1. Base contact, mostly transparent,
2. n-layer(s) (or p)
3. i-layer(s)
4. p-layer(s) (or n),
5. Top layer.
Here n or p denotes an n doping or p doping which results in an increase in the density of free electrons or holes in the thermal equilibrium state. In this context, such layers primarily represent transport layers. One or more i-layer(s) can thus consist of both a material and so-called interpenetrating networks. The light incident through the transparent base contact generates excitons in the i-layer or in the p-layer. These excitons can only be separated by very high electrical fields or at suitable interfaces. In organic solar cells, sufficiently high fields are not available, with the result that all concepts for organic cells promising success are based on exciton separation at photoactive interfaces.
The excitons reach such an active interface through diffusion, where electrons and holes are separate from each other. These can be between the p (n) layer and the i-layer or between two i-layers. In the integrated electrical field of the solar cell, the electrons are now carried away to the n area and the holes to the p area.
As excitons are always the first to be produced by the light rather than free charge carriers, the low-recombination diffusion of excitons at the active interface plays a critical role in organic solar cells. To contribute to the photoelectric current, the diffusion length in an effective organic solar cell must therefore significantly exceed the typical penetration depth of the light so that the predominant part of the light can be used. Perfect organic crystals or thin layers completely fulfill this criterion structurally and in respect to the chemical purity. For large-area applications, however, the use of mono-crystalline organic materials is not possible and the production of multiple layers with sufficient structural perfection has so far proved to be very difficult.
Instead of enlarging the excitation diffusion length, it is also possible to decrease the average gap to the next interface. WO 00/33396 discloses the formation of a so-called interpenetrating network: A layer contains a colloidally dissolved substance which is dispersed in such a way that a network forms through which charge carriers can flow (percolation mechanism). Either only one of the components or both components assume the task of the light absorption in such a solar cell. The advantage of this mixed layer is that the excitons generated only have to travel a very short distance until they reach a domain boundary where they are separated. The removal of electrons or holes occurs separately in the dissolved substance or in the remaining layer. As the materials are in contact with each other everywhere in the mixed layer, it is crucial in this concept that the separated charges have a long life on the relevant material and that closed percolation paths are available for both charge carriers to the respective contact from each location. This approach has enabled efficiencies of 2.5% to be achieved [C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Advanced Functional Materials 11, 15 (2001)].
Further familiar approaches for realizing or improving the properties of organic solar cells are listed in the following:                1.) One contact metal has a large work function and the other one has a small work function, with the result that a Schottky barrier is formed with the organic layer [U.S. Pat. No. 4,127,738].        2.) The active layer consists of an organic semiconductor in a gel or binding agent [U.S. Pat. No. 0,384,4843, U.S. Pat. No. 0,390,0945, U.S. Pat. No. 0,417,5981 and U.S. Pat. No. 0,417,5982].        3.) The creation of a transport layer which contains small particles which assume the charge carrier transport [U.S. Pat. No. 5,965,063].        4.) A layer contains two or more types of organic pigments which possess different spectral characteristics [JP 04024970].        5.) A layer contains a pigment which produces the charge carriers, and additionally a material which carries away the charge carriers [JP 07142751].        6.) Polymer-based solar cells which contain carbon particles as electron acceptors [U.S. Pat. No. 5,986,206].        
17.) Doping of the above mixed systems for improvement of the transport properties in multiple layer solar cells [Patent application—file number: DE 102 09 789.5-33].                8.) Arrangement of individual solar cells on top of each other (tandem cell) [U.S. Pat. No, 4,461,922, U.S. Pat. No. 1,619,8091 and U.S. Pat. No. 1,698,092].        
U.S. Pat. No. 5,093,698 discloses the doping of organic materials: Admixing an acceptor-like or donor-like doping substance increases the equilibrium charge carrier concentration in the layer and enhances the conductivity. According to U.S. Pat. No. 5,093,698 the doped layers are used as injection layers at the interface to the contact materials in electroluminescent components. Similar doping methods are analogously suitable for solar cells.
Despite the advantages described with interpenetrating networks, a critical issue lies in the fact that closed transport paths for both electrons and holes to their respective contacts have to be present in the mixed layer. Furthermore, as the individual materials only fill a part of the mixed layer, the transport properties for the charge carriers deteriorate significantly in comparison to the pure layers.
Owing to the small exciton diffusion lengths or transport and recombination problems in interpenetrating layers, the active layer thicknesses of organic solar cells are usually lower than the penetration depths of the light. Moreover, organic dyes only exhibit individual absorption bands, with the result that a material can never cover the complete optical spectrum. It is therefore desirable to use so-called light traps (light trapping) or to be able to stack several cells over each other. Such stacked cells were first realized by Yakimov et al. [A. Yakimov, S. R. Forrest, Appl. Phys. Lett. 80 (9). 1667 (2002)]. They consist of two layers per individual cell and requisite recombination centers at the interface between the individual cells. If, like Yakimov, we apply these recombination centers directly onto the photoactive material, they will not only ensure the required recombination of charge carriers from the n-th cell with opposite charge carriers from the n+1-th cell, but also form undesired recombination centers for excitons or charge carrier pairs from one and the same cell. Either recombination losses or inactive areas result from this. To prevent these effects, the layers must be made thicker than the corresponding width of the relevant photoactive zone so that absorption occurs in areas where they cannot be used. Such a problem occurs analogously in individual diode structures. In this way, however, the recombination losses occur immediately at the transition zones between the active layer and contact electrode.